The amount of titanium metal produced has been greatly increased due to a recent feature of demand increase in the world not only in the aircraft industry, but also in the other fields. Demand for titanium sponge and titanium metal ingots have been greatly increased due to the increase of the titanium metal production.
The titanium metal ingots are produced in a vacuum arc remelting furnace by melting the titanium sponge briquette, which briquettes are formed of compacting titanium sponges produced by the Kroll Process in which titanium tetrachloride is reduced by such a reducing metal as magnesium.
The following process is also known as another process for producing titanium metal ingots, in which titanium metal scrap is mixed with titanium sponge to prepare raw material for melting, the raw material being melted by an electron beam melting furnace or a plasma melting furnace. An example of this electron beam melting furnace is shown in FIGS. 1 to 3 (FIG. 2 is a plane view of FIG. 1 seen from direction A, and FIG. 3 is a cross-sectional view taken along line B-B).
The raw material is not necessarily formed into the electrode in this electron beam melting furnace, which is different from the vacuum arc melting furnace and a granular or agglomerated raw material 12 can be fed into a melting hearth 13.
Since molten metal 20 generated by melting the raw material 12 in the hearth 13 is flowed from the hearth 13 into a mold 16, impurities in the molten metal can be removed by the vaporization of impurities in the raw material, therefore a highly pure titanium metal ingot can be produced n the electron beam melting furnace.
In this way, the electron beam melting furnace with a hearth can produce a highly pure ingot metal not only in case of titanium metal, but also in case of such a refractory metal as zirconium, hafnium or tantalum containing impurities therein.
The ingot 22 cooled and solidified in the mold 16 mentioned above is extracted by an extracting jig 30 in the electron beam melting furnace. Since the ingot 22 just after extracted from the mold 16 is kept at high temperature and the inside of extracting zone 50 is at reduced pressure, it is difficult to directly cool the ingot like a water spray cooling in a continuous casting of steel (see Japanese Unexamined Patent Application Publication No. Hei 10 (1998)-180418. From a practical perspective, as shown by wavy arrows in FIGS. 1 and 3, when the ingot 22 is cooled only by radiation of heat, it may take a very long time until the ingot temperature reaches a room temperature. As is explained, since cooling of the ingot in the extracting area 50 takes a long time, a efficient cooling apparatus of the ingot produced in the mold 16 has been desired.
As another method to improve the productivity of the melting furnace for producing metal, a technique is known in which molten metal generated by melting an electrode in one retort is poured into multiple molds which can produce simultaneously multiple ingots (see U.S. Pat. No. 3,834,447).
Furthermore, in order to improve productivity of an ingot, an electron beam melting furnace is proposed, in which molds 16 are provided, molten metal is divided by a ladle 17 to produce multiple ingots at the same time as shown in FIGS. 4 to 7 (FIG. 5 is a plane view of FIG. 4 seen from direction A, FIG. 6 is a side view of FIG. 4 seen from direction C, and FIG. 7 is a cross-sectional view taken along line B-B) (see Japanese Patent Application Laid Open No. Hei03 (1991)-75616).
As mentioned above, Ingots 22 also is cooled merely by a radiation, and thus cooling efficiency is quite low in the electron beam melting furnace. Furthermore, as shown in FIGS. 6 and 7, the heat content of the ingot is removed appropriately by a radiation from the ingot surface to the outer case 51 in the extracting zone; however, the extent of the heat radiation is decreased in case that the ingot surface is mutually faced each other (near the central area in the extracting area 50), and as a result, the cooling rate of the ingot is decreased.
Furthermore, non-uniform temperature distribution in an ingot may cause deformation of the ingot such as warping or curving. Thus these problems should be solved.
A so-called “solidified shell” like a skin solid is formed on the mold inner surface contacting the molten metal in the mold pool. The thickness of the solidified shell has a tendency of the increase toward the bottom part of the mold pool and then the mold pool region is decreased and only the solid ingot is remained in the lower portion of the mold. This is because the amount of heat loss toward the bottom of the mold is increased in addition to the amount of the heat loss to the mold side wall.
An interface boundary between the mold pool and the solidified shell often figures a parabolic line on a cross sectional area along a vertical direction as shown by reference numeral 21b in FIG. 31A. The thickness of the solidified shell formed on an inner wall surface of the mold has a tendency to increase toward vertically the lower direction of the mold pool. This results in decreasing the mold pool region, decreasing stirring effect of molten salts by convection in the mold pool, and undesirably segregating alloy components. Therefore, as shown in FIG. 31B, it is preferable for the interface to have a parabolic shape in which a bottom parabolic line is swelled toward both sides. It is known that it is preferable that the thickness of a solidifying shell formed on the inner wall surface of the mold from the top of the mold pool to the bottom of mold pool (meniscus portion, 21a) be as constant as possible in order to maintain the casting surface of the ingot produced good condition.
As explained so far, in an electron beam melting furnace for titanium metal, an apparatus of the electron beam melting furnace having a mold in which thickness of a shell formed on an inner wall surface contacting the mold pool is kept as thin as possible, the meniscus portion is kept long, and the bottom part of the mold pool is formed wide, is desired.